The loose, shifting corners have always been an issue, plus I've never personally seen a painting keyed successfully to remove looseness.
When I learned stretching in my father's shop 25 years ago, he had lots of Frederix bars, and also strainer for custom cutting. When using Frederix, we would square it in a doorjam, then to help with the shifting-corners problem, the corners were stapled in place. With these issues, I prefer strainers. No shifting. No out-of-square issues ever. Fully customized size.
Chris- with all due respect to your father - GAP (generally accepted practices) have changed. What we knew and did 25 years ago might not be what we do today.
Paul Cascio may take me to task for "self-promotion" - but I showed in my Striplining and Stretching class some ways to overcome the "racking" you are referring to by the use of removable bracing that one can either make themselves, or purchase from a company such as Jack Richeson. I also showed several types of adjustable bars that many attendees told me they had never seen before.
I agree that it can be problematic to keep a keyable bar square, especially when yanking on a canvas with pliers - so much so that you could cause the bars to become un-square.
Have you ever used a mechanical bar such as those made by Simon Liu? I have NEVER been able to rack that type of bar - even if pushed from corner to corner.
www.simonliuinc.com
I do not like the use of staples to hold a keyable stretcher square. First of all stapling the corners in place means that either you have applied staples UNDER the canvas, thereby "fixing" the corner and rendering it un-adjustable (since you can't remove the staples from under the stretched canvas) - or you have stapled only one side - and must use a staple long enough to do some good- and thereby have stapled THROUGH the tongues of the stretcher possibly splitting them.
I know many of you won't like hearing this, but if you are pulling on a stretcher enough that you have torqued it out of square, you are probably pulling too hard on the canvas in the first place.....and I am willing to bet it involves the use of a canvas plier and staples/tacks to hold the canvas in place.
The stretching method I demonstrate (that I learned from the conservators I work with) - is gentle and allows for adjustment (sometimes it needs to be periodic and the actual restretching of a painting is accomplished over a period of days) - or involves localized "flattening" of problematic areas. "Pulling the wrinkles/ripples out" with a pliers is just bad practice.
One must also take into consideration that we are talking about "restretching" a canvas that came off bars of unknown origin. I have found that many old canvasses (canvi ?) that I have worked on were not square by the time they got to me - meaning that either the original bars were not square to begin with or warped, etc.
By using an adjustable bar, one can more accurately create a shape that duplicates the original stretcher.
Personally, I think we tend to "overstretch" in the first place.
My class has been approved by the PPFA as a CEU class for the MCPF and adheres to the PPFA guidelines for framing works of art on canvas. One may not agree with everything I teach (or what is contained in the Guidelines) - but if one wants to pass the CPF or MCPF test - or maintain either credential, the information conveyed is the most current GAP.
